1. Field of the Invention
The present invention relates to a groove shape of welded structure welded by using single butt welding and an inspection method of a weld zone for inspecting the weld zone welded by the single butt welding in a nondestructive state and particularly to a groove shape and an inspection method capable of being appropriately adapted for welding a structure having a hollow portion.
2. Description of the Prior Art
Conventionally, in carrying out a single butt welding, a welding has been carried out by working a groove shape into a shape such as square groove, single V groove, single bevel groove, single J groove, single U groove or the like. However, if a surface of a welding heat source side of a groove portion and a surface of an opposite side thereof, that is to say, a surface of a lower surface side of the groove (hereinafter, referred to as a surface of a lower surface side of a groove) remain not welded when the welding is carried out with those groove shapes, it causes a lack of strength or becomes a cause of crevice corrosion and moreover, in welding a pipe for supplying a gas used in a field of semiconductor manufacturing industry or the like, it makes very microscopic particles, such as metal particles or the like, accompanied in a supplied gas to raise a problem to produce inferior goods.
The cause of the above-mentioned problem may be expected that, for example, weld line deviation is caused by wrong establishment and manipulation in arc aiming during welding such that penetration bead does not reach a surface of a lower surface side of the groove of a butt portion and the groove remains, or a complete penetration is not accomplished due to wrong establishment of welding conditions such that the groove on the surface of the lower surface side of the groove remains.
Furthermore, in case of butt welding different kinds of metals, since an amount of complete penetration of each metal is different due to a difference in melting point of molten metal, for example, even though a welding is accurately carried out along a weld line, biased penetration where weld metal is biased to a metal of one side may be created such that the groove on the surface of the lower surface side of the groove remains to thereby cause an inferior welding.
Further, even though some metals are welded by the butt welding, since surface tension of molten metal is different due to a difference in impurity content of each metal, there is a disadvantage that weld metal is biased to a weldment of one side, similarly to the case where the different metals are welded by the butt welding.
Because of the above-mentioned problem, to determine whether there remains a flaw by the welding in the weldments or not, a state of an inside surface of a weld zone has been generally inspected by the naked eye after carrying out the welding. However, a nondestructive inspection needs to be carried out for a kind of weld structure having a hollow portion, such as a pipe, a pressurization container and a pillar having a box-shaped cross section used in a steel frame for construction, which are hard to be inspected by the naked eye.
As the nondestructive inspection method, an ultrasonic test and a radiograph test have been generally adopted. However, when a butt surface is relatively flat, even though the groove of the surface of the lower surface side of the groove remains by any cause, there is case wherein the remaining grooves stick to each other by solidification shrinkage stress of the weld zone.
Since a difference in a transmission amount of radioactive rays is small due to close adherence of a gap in a radiograph test, it is difficult to obtain a clear difference of concentration which can be used in inspection of the flaw.
Furthermore, in an inspection by the ultrasonic test, since ultrasonic wave transmits through the closely adhered groove portion, there is a problem that sufficient S/N ratio can not be obtained.
Particularly, in the ultrasonic test of the weld zone of the weldment made of an austenite-based stainless steel, there is a problem that a small noise referred to as a forest-shaped echo by coarse columnar crystal is generated to make S/N ratio to be more lowered. Further, since the forest-shaped echoes become a bigger pseudo echo if they are overlapped, it is almost impossible to distinguish it from a reflection echo which is from a fine remaining butt portion.
Furthermore, if welding is carried out by leaving an opening between root faces in order to prevent close adherence of the groove portion which remains not melted, generation of the above-mentioned problems may be decreased. However, since it takes very much time to establish equally an opening between the root faces with a high accuracy and moreover, burn through or incomplete fusion or the like are easy to be generated, very high welding technique and skillfulness have been required. Particularly, in circumferential automatic TIG welding where a thin pipe is welded without using a filler metal, such a problem becomes considerable and it is very difficult to weld with leaving a root opening so that the welding had to be carried out in a state without the root opening. Furthermore, in case that a root face is coarse, the above mentioned problem may be reduced, however, in that case, since there are many cavities on a butt portion, a weld flaw such as generation of blowhole becomes easy to be generated.
An object of the present invention taking the above-mentioned problems into account is to provide a groove shape capable of securely inspecting and estimating a flaw due to existence of the groove on a surface of a lower surface side of the groove, which is generated due to an incomplete penetration, a weld line deviation, existence of different kinds of metals or impurities, by a nondestructive inspection such as an ultrasonic test or a radiograph test, without establishing a root opening requiring a high level of welding skill in single butt welding, and another object according to the present invention is to provide an inspection method capable of securely inspecting and estimating the flaw of a single butt weld zone.